In a disk drive, a magnetic recording head is made of read and write elements. The write element is used to record and erase data bits arranged in circular tracks on the disk while the read element plays back a recorded magnetic signal. The magnetic recording head is mounted on a slider which is connected to a suspension arm, the suspension arm urging the slider toward a magnetic storage disk. When the disk is rotated the slider flies above the surface of the disk on a cushion of air which is generated by the rotating disk.
The read element is generally made of a small stripe of multilayer magnetic thin films which have either magnetoresistance (MR) effect or giant magnetoresistance (GMR) effect, namely which changes resistance in response to a magnetic field change such as magnetic flux incursions (bits) from magnetic storage disk. Recorded data can be read from a magnetic medium because the external magnetic field from the recorded medium (the signal field) causes a change in the direction of magnetization in the read element, which in turn causes a change in resistance in the read element and a corresponding change in the sensed current or voltage.
FIGS. 1 and 2 illustrate examples of a conventional composite type thin-film magnetic head 10. FIG. 1 is a cross-sectional view of the head 10 perpendicular to the plane of the air bearing surface (ABS). FIG. 2 shows the slider 11 flying above the disk 13.
In these figures, the reference numeral 12 denotes a substrate, 15 denotes an undercoating, 20 denotes a lower shield layer of the MR reproducing head part, 21 denotes an upper shield layer of the MR head part, which can also act as a lower pole of an inductive recording head part, 22 denotes a MR layer provided through an insulating layer 23 between the lower shield layer 20 and the upper shield layer 21, 26 denotes a write gap layer, 27 denotes a lower insulating layer deposited on the upper shield layer 21, 28 denotes a coil conductor formed on the lower insulating layer 27, 29 denotes an upper insulating layer deposited so as to cover the coil conductor 28, 30 denotes an upper pole, and 34 denotes a pad that would connect the read or write coil to other components in the drive. In general, there would be a plurality of pads 34 on the slider 11. Note that the pad 34 connects directly to the coil conductor 28. The upper pole 30 is magnetically connected with the lower pole (upper shield layer) 21 at its rear portion so as to constitute a magnetic yoke together with the lower pole 21.
As recording density and data transfer rate have increased over the past a few years, critical dimensions in the recording device such as track width read and write gap and coil size have decreased accordingly. Also, the fly height between the air bearing surface (ABS) 32 and the media have become smaller and smaller. For reference, recording heads with 40 GB/in2 products typically have fly heights of about 12 nanometers. This fly height will continue to decrease in the future. This reduction in head critical dimensions and fly height, while beneficial to magnetic performance, also comes with cost on thermal and mechanic reliability.
There are several factors that limit the reduction in slider flying height. These factors might reasonably be ignored at flying heights of above 20 nanometers, but would become major concerns at flying heights on the order of <5 nanometers. These include variations in the sliders themselves, variations in the structure that supports the sliders, and media surface roughness.
Write- and temperature-induced protrusion causes variation in the fly height, typically requiring an increase in the magnetic spacing to prevent reliability issues of the head-to-disk interactions. The thermal expansion coefficients for the substrate and the various layers of the head differ, so when the head becomes heated with changes of the ambient HDD temperature, some layers will begin to protrude from the ABS. FIG. 2B depicts the head 10 when the write element is not operating. FIG. 2C is a detailed diagram of the heat transfer and protrusion profile of the head 10 when the head is active (e.g., when the write coil is energized). One issue with heads is that the write-induced protrusion of the pole and overcoat can cause head-media contact, resulting in errors and impacting reliability of the drive operation. In older generations of heads, this was not a problem because the head was flying much higher than the changes induced by write-induced protrusion. Ohmic heating from write current through the coil and eddy current in write pole/yoke and magnetic hysteresis of magnetic materials are confined in a tiny space near the ABS, which typically lead to unacceptable thermal protrusion and drive reliability concerns. As can be seen in FIG. 2C, the top write pole 30 and overcoat protrude from the ABS 32 toward the media 13. The protrusion amount is typically 1–6 nanometers.
Thermal effects also are exaggerated by minute slider flying heights. Thermal effects include the natural tendency of materials to expand when heated, quantified by a temperature coefficient of thermal expansion more conveniently called a thermal expansion coefficient. Materials with higher coefficients expand more in response to a given temperature increase. When materials having different thermal expansion coefficients are contiguous and integral, their differing expansion when heated leads to elastic deformations and elastic restoring forces in both of the materials. Reduced flying heights increase the need to take thermal expansion and thermally induced elastic deformation into account.
Normal tolerances in slider fabrication lead to structural variations among the sliders in any given batch. Consequently, the flying heights of sliders in the batch are distributed over a range, although the flying height of each slider individually is substantially constant.
Variations in supporting structure occur primarily in the transducer support arm, the suspension or gimballing structure, slider geometry and load arm. These variations influence the flying height, and the nature of a given slider's reaction to any disturbances, e.g. due to shock or vibration.
Disk roughness also becomes more of a problem at lower slider flying heights. With maximum peaks more likely to protrude into a normal range of slider operation, the probability of unintended and damaging slider/disk contact increases. The risk of damage from these discontinuities is even greater at lower slider flying heights.
Hard disk drives have to operate also at different altitudes. Changes in atmospheric pressure due to altitude variations induce changes of the flying height of the sliders, typically decreasing at higher altitude. To avoid head-disk contact, the ABS's are designed to fly slightly higher at normal altitudes to accommodate changes at higher altitudes.
One proposed design of a slider would drag on the disk surface, thereby more precisely fixing a head/disk spacing based on a peak roughness of the disk surface. Any improvement in setting the transducer/recording surface gap, however, would be at the cost of excessive wear to the slider, media recording surface, or both.
Another proposed design uses a heater inside the head structure to induce thermal protrusion, as described in U.S. Pat. No. 5,991,113 to Meyer et al. By adjusting the current into the heater, a controlled increase in the head temperature can be obtained, resulting in the protrusion of the head elements towards the disk, thereby controlling the magnetic spacing. There are many disadvantages of this design. First, the temperature of the read element is increased, thereby effecting its reliability and maximum allowable bias current. In order to provide a sufficient protrusion adjustment, significant heat must be applied to the head. This results in heating of the sensor as well, which ultimately leads to deterioration of the sensor materials. Further, because the sensor materials are heated, their conductivity is reduced, resulting in less current being able to pass through the sensor, and consequently, less signal. Second, the thermal response is slow (˜200 microseconds (μsec)), limiting applications of this design to a slow adjustment of the flying height. To compensate for various change in magnetic spacing it is desirable to have a fast adjustment of protrusion to obtain the optimal signal. Thus, the protrusion needs to be precisely timed. Third, this design requires relatively high power consumption (>25 mW) in order to produce adequate fly height adjustments. This is undesirable in computing environments requiring battery power, such as in laptop computers. Fourth, the head elements can only be brought closer to the disk; magnetic spacing cannot be increased.
What is therefore needed is a structure providing controllable protrusion while avoiding the aforementioned disadvantages.